The present invention relates generally to x-ray tubes and, more particularly, to a hard coating and lubricant deposited on an x-ray tube bearing assembly.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for the purpose of distributing heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which includes tool steel ball bearings and tool steel raceways.
Bearings used in x-ray tubes are required to operate in a vacuum, which precludes lubricating with conventional wet bearing lubricants such as grease or oil. X-ray tube bearing rolling elements are typically coated with a solid layer, or tribological system, of a metal with lubricating properties, such as silver, lead, or lead-tin. Silver, applied by an ion plating or an electroplating process, has been used as a lubricating coating for tool steel bearings in x-ray tube applications where the tubes operate under vacuum and at temperatures in the range of 300-500 degrees Celsius. The performance of the silver coating is optimum at an operating stress level of up to 2.5 GPa and a temperature of 400 to 500 degrees Celsius. Failure of a bearing in an x-ray tube is typically by wear of the plated silver and loss of the silver from the contact region.
Silver is also used because of its electrical characteristics. Tube current flows in the x-ray tube from cathode to anode as an electron beam. The tube electrical circuit requires tube current to flow through the bearing assembly, and as such, the current flows through the rolling contact points of the bearing. The electrical circuit may include the races, the balls, and any lubricant or other material that is deposited on the bearing assembly or its components to enhance the life of the bearing. As such, the tribological system on the balls or races must be sufficiently electrically conductive in order for the x-ray tube to operate.
Silver derives its lubricity from the fact that it is a highly ductile single phase noble metal. This property is dependent on operating at temperatures above the recrystallization temperature of silver, which is 0.4 to 0.5 times the melting point of silver. Therefore, silver is not as effective for bearing lubrication when operating below these temperatures, and other soft metals such as Pb and combinations of Pb and Sn have instead been used to lubricate ball bearings in x-ray applications.
Silver lubricant distributes between the balls and races during initial processing and operation of the x-ray tube to form a thin coating on the rolling contact region. The thin silver coating serves as a lubricant during the life of the bearing. Once the silver coating is worn, wear of the base material commences, which leads to increased noise, failure of the lubricant, and which can ultimately lead to catastrophic failure of the bearing. Furthermore, micro-welding may occur at contact points between balls and raceways.
The operating conditions of newer generation x-ray tubes have become increasingly more aggressive in terms of stresses because of g forces imposed by higher gantry speeds and higher anode runspeeds. As a result there is greater emphasis in finding materials solutions for improved performance and higher reliability of the bearing tribological system under the more stringent operating conditions.
Therefore, it would be desirable to have a method and apparatus to improve reliability of the lubricant and the base material in the rolling contact region and to improve the useful life of the x-ray bearing.